Escherichia coli

3.0 Application of the guideline

E. coli is the definitive indicator of recent faecal contamination in drinking water systems and is therefore a good indicator of the possible presence of enteric pathogens of human health concern. Consequently, detection of E. coli in any drinking water system is unacceptable.

3.1 Public drinking water supply systems

3.1.1 Testing requirements

Residual disinfectant and turbidity should be determined on a daily basis as a minimum in water leaving a treatment plant. These recommendations do not apply to systems served by groundwater of excellent quality where disinfection is practised to increase the margin of safety. Where possible, daily testing for disinfectant residuals and turbidity should be supplemented with at least weekly tests for E. coli to confirm microbiological safety. In public supply distribution systems, the number of samples collected for E. coli testing should reflect the size of the population being served, with a minimum of four samples per month. The actual sampling and testing frequencies for E. coli, residual disinfectant, and turbidity in treated water entering and within distribution systems will be prescribed by the responsible authority.

3.1.2 Notification

If E. coli is detected in a public drinking water system, the system owner should immediately notify the responsible authorities and resample and test the positive site(s). A quantitative method, as opposed to a presence-absence (P-A) test as often done initially, is suggested for re-analysis, as it provides useful information on the level of contamination. If resampling and testing confirm the presence of E. coli in drinking water, the owner of the waterworks system should immediately issue a boil water advisory^* in consultation with the responsible authorities, carry out the corrective actions described below, and cooperate with the local responsible authority in any surveillance for possible waterborne disease outbreaks (see Appendix A: Decision Tree for Routine Microbiological Testing of Public Systems ). Depending on the extent of E. coli contamination in the first sampling -- for example, positive sample results from more than one location in the distribution system -- the owner or the responsible authority may decide to notify consumers immediately to boil their drinking water or use a safe alternative source and initiate corrective actions without waiting for confirmation.

3.1.3 Corrective actions

If the presence of E. coli in drinking water is confirmed, the owner of the waterworks system should carry out appropriate corrective actions, which could include the following measures:

• Verify the integrity of the treatment process and distribution system.
  
  
• Verify that the required disinfectant residual is present throughout the distribution system.
  
  
• Increase chlorine dosage, flush water mains, clean treated water storage tanks (municipal reservoirs and domestic cisterns), and check for the presence of cross-connections and pressure losses. Water should be dechlorinated before being discharged to fish-bearing streams. The responsible authority should be consulted regarding the methods available, and the correct procedure, for carrying out dechlorination.
  
  
• Sample and test the positive site(s) and locations adjacent to the positive site(s). Tests performed should include E. coli, total coliforms, disinfectant residual, and turbidity. At a minimum, one sample upstream and one downstream of the original sample site(s) plus the finished water from the treatment plant as it enters the distribution system should be tested. Other samples should be collected and tested following a sampling plan appropriate for the distribution system.
  
  
• Conduct an investigation to identify the problem and prevent its recurrence, including a measure of raw water quality (e.g., bacteriological, turbidity, colour, conductivity) and variability.
  
  
• Continue selected sampling and testing (e.g., bacteriological, disinfectant residual, turbidity) of all identified sites during the investigative phase to confirm the extent of the problem and to verify the success of the corrective actions.

If a boil water advisory is issued, it should be rescinded only after a minimum of two consecutive sets of samples, collected 24 hours apart, show negative results demonstrating full system-wide integrity (including acceptable microbiological quality, turbidity, and/or disinfectant residuals). Additional negative results may be required by the local responsible authority. Further information on boil water advisories can be found in Health Canada's Guidance for Issuing and Rescinding Boil Water Advisories (Health Canada, 2001). Only a history of data together with the verification of the suitability of the system design and its operation and maintenance can be used to confirm the long-term integrity of a supply.

Barring system-specific exemptions, all public supplies should be disinfected to produce microbiologically safe water and a disinfectant residual should be maintained throughout the distribution system at all times. In addition, all public supplies derived from surface water sources and groundwater under the direct influence of surface water should be treated in accordance with the guideline technical document for Turbidity (Health Canada, 2003).

3.2 Semi-public^** and private drinking water systems

3.2.1 Testing requirements

Testing frequencies for semi-public systems will be determined by the responsible authority and should include times when the risk of contamination is greatest -- for example, spring thaw, extended heavy rains, or dry periods. Owners of private supplies should be encouraged to have their water tested during these same periods. New or rehabilitated wells should also be tested before use to confirm microbiological safety.

3.2.2 Notification

The presence of E. coli in a semi-public or private drinking water system demonstrates that the source or the system has been impacted by recent faecal contamination; as a result, the water is unsafe to drink. The drinking water should be immediately retested to confirm the presence of E. coli. The responsible authority should advise the owner to boil the drinking water or to use a safe alternative source in the interim. If resampling confirms that the source is contaminated with E. coli, the corrective actions described below should be taken immediately. As a precautionary measure, some jurisdictions may recommend immediate corrective actions without waiting for confirmatory results (see Appendix B: Decision Tree for Routine Microbiological Testing of Semi-Public and Private Systems ).

3.2.3 Corrective actions for disinfected supplies (surface water supplies and groundwaters under the direct influence of surface waters)

The first step, if it has not already been done, is to conduct a sanitary survey to verify the safe condition of the drinking water system as applicable, including water intake, well, well-head, pump, treatment system (including chemical feed equipment, if present), plumbing, and surrounding area.

Any identified faults should be corrected before proceeding. If all the physical conditions are acceptable, some or all of the following corrective actions may be necessary:

• Verify that a disinfectant residual is present throughout the system.
  
  
• Increase the chlorine dosage, flush the system thoroughly, and clean treated water storage tanks and domestic cisterns. Water should be dechlorinated before being discharged to fish-bearing streams. The responsible authority should be consulted regarding the methods available, and the correct procedure, for carrying out dechlorination.
  
  
• Retest to confirm that the water is safe to drink

If the problem cannot be corrected, additional treatment or a new source of drinking water should be considered. In the interim, any initial precautionary measures should continue; for example, drinking water should continue to be boiled or an alternative safe source of water should continue to be used.

Barring system-specific exemptions, all semi-public supplies should be disinfected to produce microbiologically safe water. Responsible authorities may also recommend disinfection of private supplies. In addition to disinfection, semi-public and private supplies derived from surface water sources or groundwater under the direct influence of surface waters should receive adequate filtration (or use technologies achieving equivalent quality). Drinking water taken from pristine surface water sources may be exempt from the filtration requirements (Health Canada, 2003).

3.2.4 Corrective actions for non-disinfected wells

The first step, if it has not already been done, is to conduct a sanitary survey to verify the safe condition of the well, well-head, pump, plumbing, and surrounding area.

Any identified faults should be corrected before proceeding. If all the physical conditions are acceptable, then the following corrective actions should be carried out:

• Shock chlorinate the well and plumbing system. Further information on this topic is available in Health Canada's What's in Your Well?--A Guide to Well Water Treatment and Maintenance (Health Canada, 2004).
  
  
• Flush the system thoroughly and retest to confirm that the water is safe to drink. Confirmatory tests should be done no sooner than either 48 hours after tests indicate the absence of a chlorine residual or 5 days after the well has been treated. Local conditions may determine acceptable practice. Water should be dechlorinated before being discharged to fish-bearing streams. The responsible authority should be consulted regarding the methods available, and the correct procedure, for carrying out dechlorination.

If the water remains contaminated following shock chlorination, further investigation into the source of the contamination should be carried out. If the source cannot be found or cannot be corrected, either an appropriate disinfection device or well reconstruction or replacement should be considered. Drinking water should be boiled or an alternative safe source of water should continue to be used in the interim.

It should be noted that a boil water advisory should be rescinded only after a minimum of two consecutive sets of samples, collected 24 hours apart, show negative results. Further information on boil water advisories can be found in Health Canada's Guidance for Issuing and Rescinding Boil Water Advisories (Health Canada, 2001). An additional test should be taken after 3-4 months to ensure that the contamination has not recurred. Only a history of data can be used to confirm the long-term integrity of a supply when applied jointly with sanitory surveys. Further information on routine monitoring can be found in section 8.0, Sampling for E. coli.

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^* For the purpose of this document, the use of the term 'boil water advisory' is taken to mean advice given to the public by the responsible authority to boil their water, regardless of whether this advice is precautionary or in response to an outbreak. Depending on the jurisdiction, the use of this term may vary. As well, the term 'boil water order' may be used in place of, or in conjunction with, 'boil water advisory.'

^** For the purposes of this document, a semi-public water supply system is defined as a system with a minimal or no distribution system that provides water to the public from a facility not connected to a public supply. Examples of such facilities include schools, personal care homes, day care centres, hospitals, community wells, hotels, and restaurants. The definition of a semi-public supply may vary between jurisdictions.